1. Field of Invention
This invention relates to a method and apparatus for improving the structure of saturated masses of granular materials and more particularly to the prevention of liquefaction by actively withdrawing water from the granular mass while applying a vibratory force.
2. Description of Related Art
The concept of improving the engineering behaviour of weak and compressible sand deposits by inserting a vibrator into the ground has been used since the 1930s. It has been common practice since then to add water while applying vibration within the ground. This approach is commonly called xe2x80x9cVibroflotationxe2x80x9d.
In the past, the inventor has proposed a vibrodrain approach as a technical improvement on vibroflotation. In his Canadian patent No. 1,338,305 issued May 7, 1996, the disclosure of which is incorporated herein by reference, the inventor describes how deliberately extracting water from a granular mass, rather than adding water to the mass, at the same time it is being subjected to vibration, gave a better result in terms of improving the structure of the granular material. These masses of granular material can be either natural deposits of sands and silts; industrial wastes such as mine tailings or other two phase accumulations of discrete particles. The benefits associated with the vibrodrain technique include a reduction in the volume of the mass, increasing the shear strength of the mass and decreasing the compressibility of the mass. The above benefits can be achieved at virtually any depth within the body of the mass. By way of example, the vibrodrain technique finds particular application in the treatment of loose soils to prevent ground liquefaction during earthquakes such as occurred in the Marina district of San Francisco during the 1989 Loma Prieta earthquake. Ground improvement according to the vibrodrain technique would have avoided or reduced damage and losses. The vibrodrain technique is not limited to providing beneficial treatments to the materials and environment discussed above, but this provides a significant and readily understandable example of the benefits of the technique.
The inventor has previously developed tools to carry out the vibrodrain technique described above. These previous tools are disclosed in his U.S. Pat. No. 5,282,699 issued Feb. 1, 1994 and entitled Method and Apparatus for Densification of Sands or Silts, the disclosure of which is incorporated herein by reference.
Currently available vibrodrain tools rely on two separate modules, one specifically for producing vibrations, another for withdrawing water from the ground. The drain is stacked on top of the vibrator with the intake of the drain some seven feet above the centre of the vibrator. Vibration is achieved by rotation of an eccentric weight. High pressure air flow is used to purge the drainage water from the system utilizing the venturi effect. Using air rather than a mechanical pumping system allows grit laden water to be handled with reduced wear of components.
Current vibrodrain tools suffer from the following constraints on performance:
The fact that the drain intake is situated above the vibrator means that once the drainage module rises above the watertable, the benefit of concurrent drainage and vibration is lost. This means that the top seven feet or so of saturated ground do not receive the full benefits of the vibrodrain technique.
In cases where water flow rates are low, expansion of the high pressure air flow delivered as highly compressed air causes freezing of the water resulting in blocking of the outlet conduit used to drain away water, thereby essentially nullifying, or severely curtailing the benefits of drainage.
To address the above shortcomings, the present invention provides a novel vibrodrain tool and method that employs a single module for both producing vibrations and for withdrawing water from the ground. The vibrodrain tool of the present invention places the drainage intake at substantially the same level at the vibrator so that the influence of vibration and drainage are concentrated in the same area.
Instead of relying on the venturi effect to drain water, the vibrodrain tool and method of the present invention rely on a mechanical impeller system to extract water from a granular mass during vibration of the region.
In accordance with one aspect of the invention, there is provided a method of improving the structure of saturated masses of granular materials. The method may involve generating and applying within a region of said mass a vibratory compacting force generally adjacent a source of the force and during generation and application of said force, activating an impeller blade adjacent the source of the vibratory force to produce a low hydraulic pressure sink to draw water from said region into a space generally adjacent the source of the vibratory force.
In accordance with another aspect of the invention there is provided an apparatus for improving the structure of saturated masses of granular materials, comprising:
a) an axially elongated probe having a generally cylindrical side wall and upper and lower ends, the lower end being adapted to penetrate the mass, the side wall having a screen portion to restrict the passage of particles therethrough, and to pass water therethrough into a space within the probe;
b) a vibrating member located within the probe and generally adjacent the screen portion to generate a vibrating force within a region of said mass;
c) at least one impeller blade located within the probe generally adjacent the vibrating member to generate a low pressure zone to draw the water from said region into the space;
d) a discharge conduit communicating with the space associated with the screen portion, the conduit extending upwardly to the upper end of the probe;
e) an air-lift pump located at the upper end of the probe in communication with the discharge conduit to expel water from the space; and
a motor located within the probe and generally adjacent the vibrating member to power the vibrating member and the at least one impeller blade.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.